In particular, the present invention refers to a method for attenuating the oscillation of a ship through stabilizing fins or other slewing devices which perform their function through a rotation.
Systems and facilities are known to decrease pitch, roll, jolt, undesired ship movements, in addition to apparatuses for measuring the oscillating behaviour of the ship.
Generally, an anti-roll device for a ship comprises at least due fins connected to the ship hull, which are capable of reducing ship motions at a zero ship speed, also called anchored ship, and for a navigating ship. The anti-roll device is such that its fins comprise at least one mobile hydrodynamic appendix.
Several shapes of fins are known, among which those with two axes.
In particular, patent EP2782822B1 discloses a device for actively stabilizing a ship, wherein the ship in a first operating state moves and in a second operating state stops, or is anchored. The device comprises at least one element with a wing profile connected to a drive. Such element with wing profile is connected to the ship hull through a hinge mechanism which is configured for rotating the element with wing profile, through a rotation around a first and/or a second rotation axis, from an inactive position, wherein at least one surface of the element with wing profile is substantially parallel and near to the external side of the ship, to an operating position, wherein the element with wing profile is projecting with respect to the external side of the ship.
The prior art is also given by WO2009083892A2 and EP1498348A1.
Both EP2782822B1, and the prior art in general do not deal with the problem of the negative contribution of decelerating effects, of a relevant order of magnitude with respect to accelerating and viscous effects deriving from the rotation of the stabilizing fins, above all under null speed conditions of the ship, or anchored ship.
As will be explained below in the following description of the present invention, FIG. 1 schematically shows a ship with the right fin. Only the right fin is shown, with sizes and positions related to the ship, as an example. The fin is shown in the null attack angle position. The concept of attack angle, valid when the ship is navigating, is used to identify the zero position of the fin.
FIG. 2 shows the use of the fin, according to the prior art, for the anchored stabilization. With respect to the zero position schematically shown in FIG. 1, the fin is rotated by a maximum angle of ±60° around the zero position. The rotation of the fin, with zero ship speed, is capable of developing a roll moment which can reduce the ship motions.
The stabilization through fins of the anchored roll, or a zero or low ship speeds, uses the fins through rotation with a maximum angle range of about ±60° with respect to the zero position.
The fin rotation speed generates a force perpendicular to the fin. This force produces a roll moment on the ship which is proportional to the cosine of the fin angle measured with respect to zero.
The angle assumed by the fin is null when the fin is parallel to the main axis of the ship, as shown in FIG. 1.
The generated roll moment, assuming a rotation of the fin at constant speed on all 360° which produces a constant force, is proportional to the cosine of the fin angle. Being the cosine null at ±90° of the fin angle, the generated roll moment in these points is null, and in these points the rotation of the fin generates a prow moment. The roll moment generated by the rotation of the fin is maximum for fin angles equal to 0° and to 180°.
FIG. 3 shows the side view of the right fin of a ship, right prow and left stern on the drawing. The curve shows the multiplying factor, cosine of the fin angle: such factor is maximum for a fin angle equal to 0° and to 180° and is null when the fin assumes angles +90° and −90°. The curve as function of the fin angle represents, with a polar diagram, the multiplying factor due to the fin angle in creating the roll moment. The roll moment generated by the rotation of the fin, when this latter one perform an excursion, from a starting angle to a final angle, to be able to perform the anchored stabilization, as non-limiting example, in a fin angle range of ±30° and a stroke between −30° and +30°, depends on the force of the generated fin, while the fin performs the stroke with a first acceleration step, a second step at constant speed and a third deceleration step. Such generated force is approximately of the shape in FIG. 4.
In the acceleration step A, and in the deceleration step C, the inertial forces prevail on the resistance forces, namely the force generated by the fin in step B made at constant speed. The inertial force in the acceleration step A is of the same sign as the resistance force B, at constant speed; the inertial force developed by the fin in the deceleration step C, has an opposite sign to the resistance force, step B at constant speed. Being higher than the resistance force, the global force, in the deceleration step, has an opposite sign to the other two steps. Therefore, if the first two steps A, B, acceleration and constant speed, provide a roll moment which brakes the roll motion of the ship, the last deceleration step C provides a roll moment which contributes to increase the roll itself, and is therefore harmful as regards the roll stabilization.
FIG. 5 shows the approximate behaviour of the roll moment with a fin stroke from −30° to +30°. The fin stroke is approximate and not limiting; there is a similar shape with a stroke from −40° to +40° and for a stroke from −60° to +60°.
The roll moment is obtained from the force developed by the fin multiplied by the arm and multiplied the cosine of the fin angle itself. Depending on the fin angle, the roll moment, generated by the fin, in the stroke between −30° and +30° is as shown in FIG. 6.